As environmental legislation in the United States and Canada has required that the sulfur content of diesel fuel be less than 15 ppm, the reduction in the sulfur content of diesel fuel has resulted in lubricity problems. It has become generally accepted that the reduction in sulfur is also accompanied by a reduction in polar oxygenated compounds and polycyclic aromatics, including nitrogen containing compounds, which is responsible for the reduced boundary lubricating ability of severely refined (low sulfur) fuels. While low sulfur content does not in itself cause lubricity problems, it has become the measure of the degree of refinement of the fuel, and this reflects the level of the removal of polar oxygenated compounds and polycyclic aromatics including nitrogen-containing compounds.
It has been found that low sulfur diesel fuels increase the sliding adhesive wear and fretting wear of pump components such as rollers, cam plate, coupling, lever joints and shaft drive journal bearings.
Nevertheless, concern for the environment has resulted in moves to significantly reduce the noxious components in emissions when fuel oils are burnt, particularly in engines such as diesel engines. Attempts are being made, for example, to minimize sulfur dioxide emissions by minimizing the sulfur content of fuel oils. Although typical diesel fuel oils have in the past contained 1% by weight or more of sulfur (expressed as elemental sulfur) it is now required to reduce the level to less than 15 ppm.
The additional refining of fuels oils, necessary to achieve these low sulfur levels, often results in a reduction in the levels of polar components. In addition, refinery processes can reduce the level of polynuclear aromatic compounds present in such fuel oils.
Reducing the level of one or more of the sulfur, polynuclear aromatic or polar components of diesel fuel oil can reduce the ability of the oil to lubricate the injection system of the engine. As a result of poor fuel lubrication properties, the fuel injection pump of the engine may fail relatively early in the life of the engine. Failure may occur in fuel injection systems such as high-pressure rotary distributors, in-line pumps and injectors. The problem of poor lubricity in diesel fuel oils is likely to be exacerbated by future engine developments, aimed at further reducing emissions, which will result in engines having more exacting lubricity requirements than present engines. For example, the advent of high-pressure unit injectors increases the fuel oil lubricity requirement. Similarly, poor lubricity can lead to wear problems in other mechanical devices dependent on the lubrication of the natural lubricity of fuel oil.
Lubricity additives for fuel oils have been described in the art. WO 94/17160 describes an additive, which comprises an ester of a carboxylic acid and an alcohol, wherein the acid has from 2 to 50 carbon atoms and the alcohol has one or more carbon atoms. Glycerol monooleate is an example. Although general mixtures were contemplated, no specific mixtures were disclosed. While glycerol monooleate has good lubricity properties, it is also very polar and can form emulsions with fuel and water.
U.S. Pat. No. 3,273,981 discloses a lubricity additive that is a mixture of A+B wherein A is a polybasic acid, or a polybasic acid ester made by reacting the acid with C1-C5 monohydric alcohols; while B is a partial ester of a polyhydric alcohol and a fatty acid, for example glyceryl monooleate, sorbitan monooleate or pentaerythitol monooleate. The mixture finds application in jet fuels. Such high polarity fuel additives act as detergents and are only weakly soluble in fuel.
U.S. Pat. No. 6,080,212 teaches the use of two esters with different viscosities in diesel fuel to reduce smoke emissions and increase fuel lubricity. In a preferred embodiment, methyl octadecenoate, a major component of biodiesel, was included in the formula. Similarly, U.S. Pat. No. 5,882,364 also describes a fuel composition comprising middle distillate fuel oil and two additional lubricating components. Those components being (a) an ester of an unsaturated monocarboxylic acid and a polyhydric alcohol and (b) an ester of a polyunsaturated monocarboxylic acid and a polyhydric alcohol having at least three hydroxy groups.
The approach of using a two component lubricity additive was pioneered in U.S. Pat. No. 4,920,691. The inventors here describe an additive and a liquid hydrocarbon fuel composition consisting essentially of a fuel and a mixture of two straight chain carboxylic acid esters, one having a low molecular weight and the other having a higher molecular weight.
In U.S. Pat. No. 5,713,965, the synthesis of alkyl esters from animal fats, vegetable oils, rendered fats and restaurant grease is described. The resultant alkyl esters are reported to be useful as additives to automotive fuels and lubricants.
Alkyl esters of fatty acids derived from vegetable oleaginous seeds were recommended at rates between 100 to 10,000 ppm to enhance the lubricity of motor fuels in U.S. Pat. No. 5,599,358. Similarly, a fuel composition was disclosed in U.S. Pat. No. 5,730,029, comprising low sulfur diesel fuel and esters from the transesterification of at least one animal fat or vegetable oil triglyceride.